1. Field of the Invention
The present invention relates to a transmitter, more particularly to a transmitter having an automatic level control (ALC) function.
In recent years, the time division multiple access (TDMA) mode and other systems for communication not by continuous waves, but by burst signals, for example, radio satellite communication systems, have been offered for broad practical use.
In general, it is easy to perform automatic level control on a continuous wave, but as mentioned later, it is not easy to achieve accurate automatic level control all the time for burst signals. Accordingly, there is a desire for a transmitter which can perform accurate automatic level control even with respect to burst signals in the above-mentioned communication systems.
2. Description of the Related Art
As will be explained in detail later with reference to the drawings, a general transmitter having an automatic level control (ALC) function suitable for a continuous wave includes a modulator (MOD) for modulating a carrier (CR) by a baseband signal (S.sub.b), a variable attenuator (VATT) for controlling the level of the modulated wave (S'.sub.m) output from the modulator and outputting a modulated wave (S.sub.m) with a predetermined transmission power, a detector (DET) for detecting the level of the modulated wave (S.sub.m) output from the variable attenuator, a subtractor (SUB) for receiving as input the detected level output from the detector and the reference level (REF) corresponding to the predetermined transmission power and outputting an error signal (S.sub.e) between these levels, and a loop filter (LF) for feeding back the error signal (S.sub.e) to the variable attenuator with predetermined loop characteristics.
When this transmitter is handling burst signals, however, the head level of the burst signals suddenly increases. After this, stabilization is performed using a time constant of the loop filter.
To resolve this problem, the conventional transmitter, as mentioned later, was characterized by inserting an 0-th order sample hold circuit (SH) between the subtractor (SUB) and loop filter (LF). According to the conventional transmitter, as the output modulated wave (S.sub.m) an ideal burst signal waveform is obtained with no head peak.
In this conventional transmitter, an ideal burst signal waveform having no head peak is obtained as the output modulated wave (S.sub.m).
In this conventional transmitter, however, when the power is turned on or the non-signal period between burst signals becomes long, the desired burst signal waveform cannot be obtained as an output modulated wave and a head peak appears.